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Is it the server that is having the problem, or the clients, and is it wired, wireless or both?

At this point, and from a range of 4,600 miles I will go for EMP, EMF, RFI That's electro-magnetic pulses, fields and radio frequency interference.

Basically, it sounds like someone is turning something on at that time that the NIC doesn't like. This is creating some sort of interference or just "dirtying" the power supply.

Now, it could be that the NIC card is on the way out and that has made it hypersensitive, or some other bit of nearby electrical equipment that is dying and spewing out interference in the process. Given the precision of the timing there is definately a trigger mechanism?

I guess the first question is: "what happens between 10.00 and 10.15"........... a tea break perhaps?

Does the server have a dedicated, clean power supply, or does it share with other devices?

Depending on the setup, the device having the problem might be a hub, switch or router rather than the NIC.

I am almost certain that this is an electro-mechanical problem as opposed to software.

I would say that the NIC card is on the way out and something is triggering this behaviour. Sure replacing the NIC will cure the symptoms but the problem (the trigger) will still be there lurking in the background

Cat5/6 connections and a clean power supply? yeah I have seen that before

#1 The clean power supply to the computer room ran up a wall 2 inches away from a three phase HT supply going into E6 machine shop. At 16:30hrs every Friday they would power down the banks of automatics to clean them................ that produced an EMP that the AS/400 didn't like at all

#2 Brother-in-law's pig farm. The mainframe that operated the care systems and food mill was controlled by an app called "The Big Dutchman" that ran on 2 PCs in a control room about 30 feet away. The Cat5 cables were running on top of an HT cable that fed the milling machines. Whenever they started up the EMP killed the PCs.

#3 Insurance Clerk's dumb terminal wouldn't work; neither did two replacements............... I turned off her desk fan and everything worked normally. The motor was dying and generating an EMF.

So, your customer needs to check all computer cables (Cat5 and power) and see they are not in proximity to other cabling and electrical devices.

They need to think about what happens at that time of day. Does someone turn on some neon lights somewhere? does someone test an alarm system? does the time recording system report in?

I have seen "clean" power supplies and have absolute faith in industrial electricians Get them to check that there is absolutely nothing else attached to it. And "there shouldn't be" just isn't good enough.............. physically check it.

Also check the server logs for ANY messages (including information) at or just before 10:00, there might be a clue there?

I suppose there is the off chance that some software updates itself at that time, but I feel that is a bit of a long shot............... worth asking the question though?

EDIT:

Whilst you are looking at that, you might as well ask if there are any jobs scheduled for that time of day, and if the problem also happens at the weekend?